1. Field of the Invention
The present invention relates to a method and a device of friction welding of two thermoplastic elements, and more particularly to the field of friction welding of overlapping sections of a thermoplastic strip.
2. Description of Related Art
German Patent No. 44 22 147 C2 describes a device suitable for carrying out a method, whereby a strip is first passed around a package to form a loop and tightened, and then secured by at least one clamping plate pressing the strip against an abutment plate. Overlapping sections are pressed against one another and against an abutment plate by a vibrating plate driven by a vibrating drive and thus welded together. The connecting device has two clamping plates. The first of the clamping plates presses the free end section against an abutment plate after the loop has been formed, and thus securing it. The second of these two plates secures the initial section of the loop, which is still connected to the strip supply, by pressing it against an abutment plate after the strip is tightened by a tension drive. The initial section is then cut from the strip supply. The two overlapping strip sections are superimposed between the clamping plates and a vibrating plate arranged between the clamping plates pressing the two strip sections against one another and against an abutment plate. Due to the vibrating movements of the vibrating plate, heat generated by friction between the strip sections is sufficient to melt and weld the two strip sections together.
This known joining method has proven successful in practical use. However, it has some weaknesses that can lead to an inadequate bond between the two sections of strip. Since the two clamping plates are provided only for clamping one of the two strip sections, the end stop for the free end of the strip forming the loop is provided between the clamping plate for the initial section of the strip and the vibrating plate. The distance between the end stop and the vibrating plate is only a few millimeters. Since the end of the strip made of elastic plastic moves at a high speed against the end stop, a certain rebounding of the end of the strip can be observed. In an unfavorable case, the free end of the strip may rebound several millimeters, so that it rests in the area of the weld to be produced by the vibrating plate when the end section of the strip is secured by the first clamping plate. This reduces the length of the resulting weld. This reduction in length of the weld can lead to an unacceptable weakening of the joint. Thus, there is the risk of opening of the loop of strip surrounding the package under tension.
Thus, it may be advantagous and desirous to increase the reliability and stability of the welds.
The reliability and stability of the welds may be increased according to the invention by producing at least two separate welds one behind the other in the longitudinal direction of the strip, separated by an unwelded area. According to the invention a device for friction welding has incorporated means for producing such welds.
The reliability of the joint is greatly increased by producing multiple welds. Although each individual weld has approximately only the same strength as the weld known from the related art cited above, the tensile force from each end of the loop is applied only to the adjacent weld. If there are more than two welds, essentially the two outer welds absorb the tensile force. The stress on the two stressed welds is reduced approximately in half in comparison with the individual weld from the related art. If one weld is nevertheless destroyed by the tension forces, cohesion of the two strip sections is nevertheless ensured by at least one remaining weld which is still fully functional.
The proposed method of producing at least two welds separated by an unwelded area also has some major advantages in comparison with simple extension of one weld. The welds produce the highest holding force in their edge areas. As soon as a first tear has begun at one edge area of a weld, only a relatively small force need be applied for a progressive tear to result in the weld in the plane between the strip sections and thus a complete separation of the two strip sections from one another. Due to the arrangement of at least two welds separated by an unwelded area, at least one separate weld is thus created which is fully functional in the event of failure of the first weld. Paring of the first weld cannot propagate into the second weld because of their spatial separation.
An embodiment of the invention may also include more than two welds. In order to produce a sufficiently secure joint with the least possible effort and the lowest cost, two welds seem to be an optimum number in practice.
In the simplest case, the two welds may be produced by having an interruption of several millimeters in length at the center of the surface of the vibrating plate between the two clamping plates. In the area of this interruption, no compressive force is produced perpendicular to the plane of the strip, so that no welding takes place here. The two welds may be produced by two vibrating plates arranged with a distance between them. These vibrating plates can preferably be pressed independently of one another against the strip sections to be welded.
In accordance with another aspect the invention is a method for friction welding of two overlapping sections of a thermoplastic strip with a system wrapping the strip around a package. According to this method, the strip is passed around the package to form a loop whereby sections are arranged at each end of the loop. The strip is then tightened and secured by at least one clamping plate of the system in which the strip is pressed against an abutment plate of the system. Moreover, the overlapping sections are pressed together by and against the abutment plate by a vibrating plate of the system. With vibration caused by the vibrating plate, the overlapping sections are welded whereby at least two welds are produced one behind each other in longitudinal direction of the strip and separated from one another by an unwelded area of the strip.
The two vibrating plates may be arranged with a distance between them and are pressed against the strip to produce the welds.
In one embodiment of the method according to the present invention, before forming the loop, the free end of the strip is first conveyed consecutively past a first vibrating plate and a combined clamping and vibrating plate in the direction of feed, and to a strip guidance device (strip guiding frame) which guides the strip in the form of a loop around the package. After forming the loop, the free end of the strip is conveyed past a first clamping plate, past the above-mentioned vibrating plate arranged next to the first clamping plate and past the combined clamping and vibrating plate next to it, coming to rest at an end stop. Thus, after forming the loop, the free end of the strip moves between the abutment plate and the plates covered by the initial section which is connected to the strip supply, namely the first vibrating plate and the combined clamping and vibrating plate. A strip detection signal is delivered when the strip comes in contact with an end stop. This signal causes the above-mentioned first clamping plate to move toward the abutment plate in the direction perpendicular to the plane of the strip, so that the end section of the strip is clamped between the surfaces of the first clamping plate and the abutment plate. The loop is then tightened around the package by a tension force acting against the original direction of feed and is secured by the combined clamping and vibrating plate in combination with the abutment plate, with the two overlapping strip sections being clamped between the combined clamping and vibrating plate and the abutment plate.
After securing the initial section of the loop which is still connected to the strip supply, this section may be cut off close to the first clamping plate. The first vibrating plate, which is located between the first clamping plate and the combined clamping and vibrating plate, is pressed against both strip sections and against the abutment plate. Both the combined clamping and vibrating plate and the first vibrating plate are set in vibration, so that the welds are formed.
When working according to this method, the number of moving parts is not increased in comparison with the device known from the related art to produce a weld. The second clamping plate in the direction of feed is provided with the additional function of a welding device merely by connecting it to a vibrating drive. To weld the two sections of strip, the second clamping plate in the direction of feed must clamp both sections of strip, i.e., both the initial section and the end section of the loop. This yields the additional advantage that after forming the loop, the free end of the strip can be guided to an end stop located far downstream from the second clamping/vibrating plate rather than between a clamping plate and a vibrating plate. The end stop may be 40 mm, for example, away from the clamping and vibrating plate, so that even with considerable rebounding of the end of the strip, the weld produced by the clamping/welding plate may be prevented from being shortened unacceptably.
In one embodiment, both the vibrating plate and the clamping/welding plate may be connected to the same vibrating drive. This vibrating drive may move the two plates crosswise to the longitudinal direction of the strip. Since the vibrating movements run in the transverse direction, the borders and edges of the welds are not blurred by the vibration and can be formed in a well-defined manner. This reduces the danger of the edges of the welds running into one another and linking the welds together.
In accordance with yet a further aspect the invention is a method for packing a package with a packing system, whereby a thermoplastic strip is wrapped and passed around the package. The strip is then tightened and secured by at least one clamping plate of the system, pressing the strip against an abutment plate of the system. Moreover, two overlapping sections of the strip are pressed together by at least one vibrating plate of said packing system. Additionally, overlapping sections are pressed against the abutment plate by the at least one vibrating plate. The overlapping sections are connected by friction welding by the at least one vibrating plate whereby at least two welds are produced one behind each other in longitudinal direction of said strip and separated from one another by an unwelded area of the strip.
In accordance with another aspect of the invention is a device for friction welding of two overlapping sections of a thermoplastic strip. The device includes at least one clamping plate, an abutment plate working together with the clamping plate for securing the sections, a first vibrating plate which presses the sections against one another in an overlap area and against the abutment plate to produce a first weld, and a second vibrating plate arranged at a distance from the first vibrating plate. The second vibrating plate presses the sections together in the overlap area and against the abutment plate to produce at least one second weld. This device may also be used for carrying out the above-mentioned methods. Preferably, the distance between the first vibrating plate and the second vibrating plate is at least 5 mm, especially about 10 mm.
One embodiment of the device for friction welding of the overlapping sections of the thermoplastic strip further comprises at least one vibrating drive connected to at least one of the vibrating plates for driving the at least one vibrating plate, preferably for moving the at least one vibrating plate in the transverse direction of the strip. Moreover, the vibrating plates may be connected to one single vibrating drive.
The above-mentioned device may also comprise a first clamping plate driven by a clamping drive pressing the clamping plate against a first of the sections resting on the abutment plate. A second clamping plate is driven by a clamping drive pressing the second clamping plate against a second of the sections supported by the abutment plate. At least one of the vibrating plates is arranged between the first and second clamping plates. In one arrangement, the second clamping plate may be connected to a vibrating drive to form a combined clamping and vibrating plate. Moreover, the clamping plate and the vibrating plates may be supported against one single abutment plate.
In one further embodiment of the device, the abutment plate, the clamping plate and the vibrating plate may have surfaces which are structured and provided with pointed teeth.
The invention relates also to a device for packing a package, comprising means for wrapping and for passing a thermoplastic strip around said package. The strip has two overlapping sections, means for tightening the strip, at least one clamping plate, an abutment plate against which the strip is pressed by said at least one clamping plate for securing said strip, a first vibrating plate which presses said sections against one another in an overlap area and against the abutment plate to produce a first weld, and a second vibrating plate arranged at a distance from the first vibrating plate. The second vibrating plate presses the sections together in the overlap area and against the abutment plate to produce at least one second weld. This device may also be used for carrying out the above-mentioned methods.
In one embodiment of that device according to the invention, the distance between the first vibrating plate and the second vibrating plate is at least 5 mm, preferably about 10 mm. Moreover, the device may further comprise at least one vibrating drive connected to at least one of the vibrating plates for driving the at least one vibrating plate, preferably for moving the at least one vibrating plate in the transverse direction of the strip. The vibrating plate may be connected to one single vibrating drive.
In another embodiment of the invention, the device further comprises a first clamping plate driven by a clamping drive pressing the clamping plate against a first of the sections which rests on the abutment plate and is wrapped around the package and guided up to an end stop. The device also includes means for passing the strip around the package for forming a loop, a tension drive for tightening the loop formed by the strip, a second clamping plate driven by a clamping drive pressing the second clamping plate against a second of said sections supported by the abutment plate. At least one of the vibrating plates is arranged between the first and second clamping plates. The first section may be an end section of the loop. The second section may be an initial section of the loop.
The two clamping plates may be arranged so that one of them secures the free end section while the other secures the initial section of the loop which is connected to a strip supply before the initial section is severed from the strip supply. In this embodiment, two vibrating plates are provided in the area of the strip overlap between the two clamping plates.
As an alternative, the end stop may be located downstream from the second clamping plate, so that it presses the initial section and the end section of the strip loop against the abutment plate. In this embodiment, the second clamping plate is connected to a vibrating drive to form a combined clamping and vibrating plate. Moreover, the clamping plate and the vibrating plates may be supported against one abutment plate.
In one further embodiment of the above-mentioned device, the abutment plate, the clamping plate and the vibrating plate each comprise a surface facing the strip which is structured and provided with pointed teeth.
One further aspect of the invention is a device for connecting at least two thermoplastic elements, comprising clamping means for clamping the elements, abutment means working together with the clamping means for securing the elements, a first vibrating element for friction welding of the elements. The first vibrating element presses the thermoplastic elements against one another and against the abutment means to produce a first weld, and a second vibrating element for friction welding of the elements. The second vibrating element is arranged at a distance from the first vibrating element and presses the thermoplastic element together and against the abutment means to produce at least a second weld.